Now another study is raising a particularly intriguing theory, given the expanding girth of the U.S. population: that mothers who are obese or have diabetes during pregnancy — these conditions can often go hand in hand — see higher rates of autism in their children.

Researchers at the University of California, Davis, looked at 1,004 children aged 2 to 5 years enrolled in the CHARGE (Childhood Autism Risks from Genetics and the Environment) study from 2003 to 2010. The study included 517 children with autism spectrum disorders, 172 with other developmental disorders, and 315 who were typically developing.

Obese mothers were 67% more likely than mothers of normal weight and with no metabolic disorders to have a child with autism, and they were more than twice as likely to have a child with another developmental disorder — a delay in speech delay, perhaps, or a failure to reach developmental milestones at the appropriate age, according to the study published Monday in Pediatrics.

Moms of children who were not developing typically were more likely to be obese: 21.5% of mothers of children with autism and 23.8% of mothers of children with developmental delays were obese, versus 14.3% of moms of typically developing children.

Mothers with diabetes were 2.3 times more likely to have a child with developmental delays; they were also more likely to have a child with autism, although the difference was not considered statistically significant — 9.3% of mothers of children with autism had diabetes during pregnancy compared with 6.4% of mothers of typically developing children. Kids of mothers with diabetes also scored lower on language and communication tests compared with other children. As for children with developmental delays, 11.6% of their mothers had diabetes while pregnant.

Researchers also investigated the effective of hypertension, or high blood pressure, on developmental outcomes in children, finding that prevalence was low but still higher in moms of children with autism — 3.7% versus 3.5% of moms of children with developmental delays and 1.3% of moms of typical children.

“The prenatal period is a crucial time for the etiology of autism spectrum disorders,” says Alycia Halladay, director of research for environmental sciences for advocacy group Autism Speaks, which was not involved in the study. “This adds to the body of science that there are risk factors for autism spectrum disorders that are associated with maternal health during pregnancy.”

The common thread tying together the three conditions — diabetes, obesity and hypertension — is chronic inflammation and insulin resistance, says Paula Krakowiak, lead author of the study and a Ph.D. candidate in epidemiology at UC Davis.

Krakowiak first wondered about the possible connection between autism and inflammation several years ago. Several studies had reported irregularities in the immune systems of children with autism, including findings of elevated levels of certain immune markers that indicated greater inflammation in autistic children compared with typically developing children.

She decided to look at mothers who had some level of persistent inflammation and settled upon those with diabetes, obesity and hypertension, all of which are also characterized by insulin resistance. With diabetes, bodily tissues can’t absorb enough glucose, which results in high blood sugar and causes inflammation of the blood vessels. Insulin, a hormone produced by the pancreas, isn’t released as it should be to remove the sugar from a diabetic person’s bloodstream after mealtime. In regard to obesity, having excess fat tissue creates an inflammatory, insulin-resistant environment. And hypertension, like diabetes, is marked by inflammation of the interior of blood vessels.

When a woman is pregnant, her body — in order to increase fat deposits biologically necessary during and after pregnancy — naturally becomes more insulin-resistant. “Women who are already borderline insulin-resistant before pregnancy may be more likely to develop diabetes during pregnancy because the natural processes of pregnancy make you less sensitive to insulin as the pregnancy progresses,” says Krakowiak.

Krakowiak was hesitant to proclaim that her study proves that obesity actually causes autism, but she did acknowledge that her findings raise a public health flag and warrant further research. Other studies could focus more on immunological and environmental factors potentially impacting the rise in autism. “It’s possible this is just a correlation and there is some upstream environmental factor that’s causing a rise in both metabolic conditions and autism independently,” she says. “The question of what causes autism is still open.”

Regardless of the observational findings, Halladay said the study’s conclusions reinforce the message that all pregnant women should have access to prenatal care, eat well and exercise to protect the health of their developing baby. “We know there is not one thing that causes autism,” she says. “This is one of the associated risk factors of autism, and it’s one that women can do something about — see a doctor, eat healthy and check themselves for gestational diabetes.”

Diet is particularly critical, believes Krakowiak. “It’s just another reason to take good care of yourself during pregnancy, not just for yourself but for the health of your baby,” says Krakowiak, who is pregnant.

She passed her initial gestational diabetes screen and continues to eat plenty of fruits and vegetables and steer clear of sugar. “Every once in a while I will have dessert,” says Krakowiak, who’s in her second trimester. “I’m lucky I don’t have a sweet tooth.”